|The Question: These bright orange insects were all over my deck. I have never seen anything like them. Are they ants or spiders?
Submitted by: Roxanne, Pennsylvania, USA
(click on photos and graphics to expand)
The Short Answer: Roxanne, these are neither ants nor spiders, they are the nymphs of an assassin bug, probably the wheel bug (Arilus cristatus). The assassin bugs (Reduviidae) are a family of “true bugs.” To scientists, the “true bugs” are the members of the insect order Hemiptera. The identifying trait of “true bugs” is the proboscis (also called a rostrum), a long tube through which the insect feeds.
Assassin bugs earned that name because most of the 7,000 or so species found around the world are ambush hunters, meaning they either lie in wait or try to sneak up on insects and other arthropods and then pounce at the last second. They kill their prey by piercing it with their proboscis, injecting a paralyzing venom and then sucking out the insides.
If assassin bugs were bigger, they’d make pretty good models for science fiction monsters.
Most species go for relatively undefended prey such as caterpillars, but some species will battle spiders, centipedes and even scorpions. Others specialize in one type of prey, such as millipedes. Assassin bugs will bite people (pierce, really), and although the bite is not generally dangerous, some people report that it’s more painful than a bee sting. If you can avoid picking them up and getting bitten, however, most gardeners are happy to find a newly hatched tribe of assassin bugs such as the ones you have, because assassin bugs, from the moment they hatch, are predators of other insects that we tend not to want, such as destructive caterpillars.
Newly hatched nymphs such as these, by the way, sometimes hunt in a pile-on way. If one of them finds a large prey item and pierces it, the other nearby nymphs will join in killing and draining the hapless victim.
It’s tough to tell from a photo, but if these are in fact wheel bug nymphs, they are destined to become a fairly large adult (2-3 cm, about an inch). They will have a characteristic notched “wheel” on the top of their thorax. The purpose of the wheel is not completely understood. It may have a function in the noises males make to attract females.
What’s With the Crazy Orange Color: The nymphs of many species of assassin bugs sport bright orange or red coloration. Why would a tiny insect want to be so brightly colored? Wouldn’t that make it easy prey for birds and other insect eaters?
Bright coloration among animals can evolve as a warning to predators. This is called aposematic coloration. Most people are familiar with this phenomenon in monarch butterflies or any of the many poison dart frogs of Central America. In the case of the assassin bug nymphs, it may serve as a warning to birds and others that “If you try to eat me, you will be sorry. I will give you a very painful bite.” It’s also possible that they taste bad, due to the same venom they use to paralyze prey.
The evolution of aposematic coloration relies on three things: a predator deterrent, kin selection and learning by predators.
Imagine that a genetic mutation arises that leads to a single wheel bug nymph being bright orange in coloration. Now imagine that a local bird predator sees that boldly colored nymph from far away, zeroes in on it and gobbles it. The bird may decide “Eww that tastes awful!” and be deterred from ever eating an orange bug again. The nymph, however, is already eaten, and the mutation for orange coloration will disappear with it.
But what if the mutation occurs earlier in the egg or sperm formation process so that the entire brood of nymphs end up orange colored. Now imagine that the local bird predator eats the first brightly colored nymph it sees, says “Eww!” and swears off orange colored insects forever. The nymph is probably still dead, but its death now offers extraordinary protection for all its siblings … all of whom are carrying the genetic mutation for bright orange coloration. Given that kind of situation, the orange coloration is likely to become very common since orange colored nymphs are now more likely to survive to adulthood than nymphs that are not orange colored.
This kind of adaptation, where the genetic traits of one individual benefit its close relatives, is called “kin selection.”
The same process could take place if the nymphs are surprisingly painful instead of tasting bad. Imagine that a bird tries to eat a nymph, but gets pierced inside its mouth in the process. It’s not very likely to go for those bright orange nymphs next time … unless it’s really, really hungry.
I said the evolution of aposematic coloration also requires learning on the part of the predators. That’s because if the local predator eats an orange nymph and says “Eww!” but the next time it sees an orange nymph it doesn’t remember that the previous one tasted bad, it will eat that one, too, and the siblings of bad-tasting orange nymphs won’t gain protection. Vertebrate predators like birds are certainly able to learn, and probably some invertebrate predators have learning ability as well.
So, we have a nymph that probably has a nasty taste and definitely has a nasty bite. We have large brood sizes that allow “kin selection.” And we have predators that can learn to avoid orange nymphs.
The result is startlingly day-glo orange nymphs.
This process should work just as well for adult assassin bugs, and sure enough, the adults of many species are also brightly colored and patterned, usually featuring bright red or orange. In other species, including the wheel bug, however, the bright coloration disappears by the time they are adults. Adult wheel bugs are a camouflaged grey color. It may be that for some adult assassin bugs like the wheel bug, camouflage that helps it catch prey is more important than warning coloration that helps it avoid predators.
For Your Viewing Pleasure: Here are a couple of videos that can help you gain an appreciation for assassin bugs:
This video is annoyingly overhyped with bizarre fake sound effects, but it does show a nice sequence of an assassin bug hunting a caterpillar: https://www.youtube.com/watch?v=i0NYdQ3gEwo
This one shows assassin bugs hunting different insects: https://www.youtube.com/watch?v=AE43rai4Lho
This one shows eggs and nymphs and has some good information: https://www.youtube.com/watch?v=1qBkEY6QFb4
Thanks: Thanks to bugguide.net for confirming the ID of these nymphs.
C. Weirauch et al. (2014). An illustrated identification key to assassin bug subfamilies and tribes. Canadian Journal of Arthropod Identification. No. 26.[cite]